1. Field of the Invention
The present invention relates to a virtual space image generation technique.
2. Description of the Related Art
As the processing capability of computers has improved recently, studies of a Virtual Reality technique of providing real experiences to users have progressed (see non-patent reference 1). The technique is implemented by expressing virtual space by computer graphics and displaying it on an HMD (Head Mounted Display) or a wall-type display.
In this field, an element necessary for providing a high-quality experience to a user is the speed of image generation. The processing speed required for generating a virtual space image while following a user's viewpoint movement is generally supposed to be 10 to 15 frames/sec. To meet this requirement, a technique of generating an image at a high speed while maintaining expressive power higher than before has been developed.
The recent sophisticated computer parallelization and virtual space handling techniques have enabled a real-time ray tracing method which was impossible previously (see non-patent reference 2). The ray tracing method disclosed in non-patent reference 2 is particularly called real-time ray tracing and has extensively been studied. This technique allows expressions of reflection and refraction, high-speed shadow generation, and expressions of global illumination, which were difficult for a conventional rasterization method. It is therefore possible to generate a high-quality image.
As the expressive power of image generation processing rises, a calculation load necessary for obtaining a high-quality image is increasing steadily. The amount of data to be processed is also increasing to meet the desires for displaying objects in virtual space in real time. For these reasons, even when real-time ray tracing is implemented, a calculation load reducing technique is indispensable to output at a high frame rate while maintaining high expressive power.
Patent reference 1 discloses a method of increasing the efficiency of animation generation by ray tracing using a time-series correlation. An animation expresses a motion by updating the screen (frame) that changes gradually. The gradually changing screen contains a time-series correlation (coherence) such as the positional relationship of objects that should be within sight.
In image generation processing using ray tracing, generally, the time required for a search in ray tracing is the longest. In patent reference 1, using the coherence between time-series images, the result of a preceding frame is reused for a portion where no change occurs between the preceding frame and the current frame, thereby shortening the search time in the ray tracing method.    [Non-patent reference 1] “VR World Construction Technique”, supervised by Susumu Tachi, and edited by Michitaka Hirose, Baifukan, 2000    [Non-patent reference 2] Ingo Wald, Carsten Benthin, Markus Wagner, and Philipp Slusallek, “Interactive Rendering with Coherent Ray-Tracing” in Computer Graphics Forum/Proceedings of the EUROGRAPHICS 2001, pp. 153-164, 20 (3), Manchester, United Kingdom, Sep. 3-7, 2001    [Patent reference 1] Japanese Patent No. 2532055
The above method increases the time-series efficiency and speed of image generation processing by ray tracing. However, processing of generating a plurality of images of different viewpoints at once is not taken into consideration at all. For example, to make a user wearing an HMD experience virtual space, it is necessary to generate two images for right and left eyes simultaneously and present them to him/her as a stereoscopic image. In stereoscopic image generation, the right-eye image and left-eye image are generated based on different viewpoint positions and orientations, and therefore, the ray reflection directions are different. For this reason, the time-series correlation cannot be used. To generate a stereoscopic image to be presented to the user, image generation processing must be executed for each of the right and left eyes.